Non-rotating or stationary variable stator vanes are used in compressors and fans and in some turbines of aircraft gas turbine engines. In the gas turbine engine, a shaft drives a central drum, retained by bearings, which has a number of annular airfoil rows attached usually in pairs, one rotating and one stationary attached to a stationary casing. The pair of rotating and stationary annular airfoil rows is called a stage. The rotating airfoils, also known as blades or rotors (herein “rotor blades”), accelerate the fluid. The annular row of stationary airfoils, also known as stators or vanes can either be completely fixed within the casing (“fixed stator vanes”) or able to rotate about a radial or near radial axis to change the angle with which incoming axially flowing fluid intersects the leading edge of the airfoils (“variable stator vanes”). The fixed and variable stator vanes convert the increased rotational kinetic energy into static pressure through diffusion and redirect the flow direction of the fluid (indicated by arrow A in FIG. 1A), preparing it for the rotor blades of the next stage or directing the flow into a downstream duct.
Generally, as schematically depicted in FIGS. 1A and 1B, conventional variable stator vanes 2 in variable stator vane assemblies 4 have stems 6 through their rotational axis 8 that penetrate a hub 10 and the casing (also known as a shroud 12 (for example, of a compressor 14 (partially shown in FIG. 1A))), allowing the vanes 2 to be rotated using an actuation mechanism. The airfoil 16 of each variable stator vane 2 is disposed between spaced apart inner and outer buttons 18 and 20 of material centered about the rotational axis 8. The airfoil 16 of the conventional variable stator vane 2 includes a leading edge 22 and a trailing edge 24, a root 26 and a tip 28. The leading edge 22 of the depicted airfoil of the conventional variable stator vane 2 includes a leading edge aft sweep (encircled region “A” in FIGS. 1A and 1B) (see, also, the stippling in FIG. 5B) at the root 26 near the hub 10 (also referred to herein as “an inner endwall”) in a “hub region” and a leading edge forward sweep (encircled region “B”) at the tip 28 near the shroud 12 (also referred to herein as “an outer endwall”) in a “shroud region”. The airfoil 16 may alternatively include a neutral sweep (not shown) at the root 26 near the hub or at the tip 28 near the shroud.
The leading edge aft or neutral sweep at the root in the hub region results in endwall gaps 30a and 30c and the leading edge forward or neutral sweep at the tip in the shroud region results in endwall gaps 30b and 30d (FIG. 1B) existing between the vane and the endwalls (the hub and shroud) of the flow passageway (more particularly between the shroud and the airfoil tip (“outer endwall gaps”) and between the hub and the airfoil root (“inner endwall gaps”)). Such endwall gaps are defined both forward (“leading edge inner and outer endwall gaps”) 30a and 30b and aft (“trailing edge inner and outer endwall gaps”) 30c and 30d of the inner and outer buttons 18 and 20 (i.e., the aft swept leading edge at the root of the airfoil and the forward swept leading edge at the tip of the airfoil extend beyond a button forward edge portion of the inner and outer buttons respectively). The rotational axis and the inner and outer buttons centered thereabout may be positioned to balance the size of the leading edge inner and outer endwall gaps with the size of the trailing edge inner and outer endwall gaps, for acceptable aerodynamic and mechanical performance. Endwall gap clearance levels are set sufficiently large to avoid contact between the rotated variable stator vane and the shroud and hub. However, as there is a large pressure gradient between the pressure and suction sides of the vane, leakage flow through the endwall gaps 30a-30d between the vanes and the endwalls in the flow passageway is driven across these endwall gaps, resulting in reduced fluid turning and higher aerodynamic loss at the endwalls. Aerodynamic losses are generated as the flow leaks through the inner and outer endwall gaps, especially under the leading edge inner and outer endwall gaps 30a and 30b where the aerodynamic loadings are largest. This leakage flow also causes flow non-uniformities (i.e. wakes) at the adjacent rotor blades 42 in the compressor 14, creating additional downstream losses and exciting these blades and causing potentially damaging vibrations therein because of disturbance of the flowfield about the vanes 2 and blades 42. Additionally, the rotor blades 42 of the upstream and downstream rotors provide a vane excitation source causing vibrations that may break off the corners of the aft swept leading edge, the forward swept leading edge, or both (so-called “corner vibration modes”).
Attempts to reduce leakage when using conventional variable stator vanes with an aft swept leading edge at the root and a forward swept leading edge at the tip have included moving the rotational axis and/or inner and outer buttons completely forward such that the leading edge of the airfoil at the root and tip are substantially coextensive with the button forward edge portion. While such complete forward movement of the rotational axis and/or outer and inner buttons substantially eliminates the leading edge endwall gaps, such movement undesirably causes the trailing edge 24 of the vane to be more unsupported (overhung) and increases the trailing edge inner and outer endwall gap 30c and 30d size and accompanying leakage flow, resulting in increased mechanical risk and increased aerodynamic losses. Enlargement and shaping of the inner and outer buttons 18 and 20 to minimize endwall leakage has also been attempted with spatial limits due to the adjacent vanes, but not without disadvantage.
Thus, it is desirable to provide variable stator vane assemblies and variable stator vanes thereof having a local swept leading edge and methods for minimizing endwall leakage therewith. Endwall leakage is minimized without increasing the trailing edge inner and outer endwall gap size, thereby reducing aerodynamic losses, improving mechanical performance and robustness, and improving compressor efficiency.